Pyroelectric conversion is a conversion of heat to electricity due to properties of particular materials that become electrically polar when heated, resulting in opposite charges of static electricity within the material, i.e. the material exhibits an electric polarity. The technology is relatively new and is not yet commercially available.
A vinylidene fluoride trifluoroethylene copolymer is one such pyroelectric material. An example of the use of such a pyroelectric material is set out in U.S. Pat. No. 6,528,898 B1.
When a P(VDF-TrFE) copolymer is used for converting heat to electricity, the phase transition temperature at which polarization and depolarization occurs is a function of copolymer temperature. The phase transition temperature also increases with increased electric field on the copolymer. Thus, the power conversion process generally requires the synchronization of copolymer temperature and applied electric field. However, when commercial P(VDF-TrFE) copolymer is used for power conversion, substantial power losses occur due to internal leakage at high temperature and high voltage, resulting in increased internal conduction losses during pyroelectric conversion. The only known solution for minimization of the leakage current is a reduction of the electric field on the copolymer. However, reducing the electric field seriously limits the final net power output by restricting a voltage differential needed during power conversion.
This decrease in electrical resistivity of P(VDF-TrFE) copolymers with increasing temperature has been documented before. For instance, Olsen, R. B. et al. (Pyroelectric conversion cycle of vinylidene fluoride-trifluoroethylene copolymer, Journal of Applied Physics, Vol. 57, No. 11, p. 5036, 1985) report that the electrical conduction losses in P(VDF-TrFE) films during energy conversion became unacceptably large at high temperature. Chan, H. L. W. et al. (Thermal hysteresis in the resistivity of P(VDF-TrFE) copolymers, Ferroelectrics, Vol. 196, pp. 141-146; 1997) also report that as temperature increases from 20° C. to 140° C., the resistivity of the copolymer decreases from roughly 1014 Ωm to 108 Ωm. Neither publication discusses any solution to this problem.
There are several known methods for the preparation of a P(VDF-TrFE) copolymer that generally employ the evaporation of a solvent. For example, U.S. Pat. No. 3,833,503 teaches a process of producing a stable pyroelectric element composed of a vinylidene fluoride resin. A solution of vinylidene fluoride resin is dissolved in a solvent at a temperature higher than the crystal melting point of the resin and the solvent is removed by heating or pressure reduction.
U.S. Pat. No. 4,173,033 teaches a polymeric dielectric composition of a copolymer of vinylidene fluoride and trifluoroethylene. Trifluoroethylene and vinylidene fluoride are charged into a pressure resistant reaction vessel having trifluorotrichloroethane and di-(3,5,6-trichloroperfluorohexanoyl) peroxide maintained below 0° C. The reaction vessel is immersed into a water tank of 20° C., whereby the polymerization is initiated. A vinylidene fluoride-trifluoroethylene copolymer is recovered as a white clump, which is pulverized in water by the aid of a mixer and dried to produce fine granules.
U.S. Pat. No. 6,605,246 teaches an electrical device that includes a layer of processed ferroelectric polyvinylidene fluoride polymer. Preparation of the polymer film is effected by a melt pressing method or a solution casting method that involves dissolving P(VDF-TrFE) in methyl ethyl ketone and heating the solution to evaporate the solvent.
U.S. Pat. No. 3,193,539, Process for Polymerizing Vinylidene Fluoride, issued on Jul. 6, 1965, discloses a process for polymerizing vinylidene fluoride involving charging an autoclave with water and a catalyst prior to introducing vinylidene fluoride. After sealing and shaking the mixture, the autoclave is cooled, vented and opened. The contents consist of precipitated polyvinylidene fluoride suspended in a liquid phase, which is subsequently filtered and washed with methanol.
El-Hami, Khalil and Matsushige, Kazumi (Reduction and Separation of Carbon Nanotube Bundles using the P(YDF-TrFE) Copolymer: Nanobundles, Journal of Composite Materials, Vol. 38, No. 16, August 2004) disclose a process to improve the assembly of single-walled carbon nanotubes (SWCNTs) through preparation of a nanocomposite matrix by mixing SWCNT and P(VDF-TrFE) copolymer. The SWCNT is dispersed in ethanol while the P(VDF-TrFE) copolymer is dissolved in methyl ethyl ketone. The two solutions are mixed together to form a homogeneous solution.